1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a gettering technique and a semiconductor device obtained by the manufacturing method. More particularly, the present invention relates to a method of manufacturing a semiconductor device using a crystalline semiconductor film produced by adding a metal element having catalysis to crystallization of a semiconductor film and a semiconductor device.
2. Description of the Related Art
A thin film transistor (hereinafter referred to as a TFT) is known as a typical semiconductor element using a semiconductor film having a crystalline structure (hereinafter referred to as a crystalline semiconductor film). The TFT is noted as a technique for forming an integrated circuit on an insulating substrate made of glass or the like, and a driver circuit integrated liquid crystal display device and the like are putting into practical use. According to a conventional technique, an amorphous semiconductor film deposited by a plasma CVD method or a low pressure CVD method is processed by heat treatment or a laser anneal method (technique for crystallizing a semiconductor film by laser light irradiation) to produce the crystalline semiconductor film.
Since the crystalline semiconductor film thus produced is an aggregate of a large number of crystal grains, and its crystal orientation is oriented in an arbitrary direction which is thus uncontrollable, this causes a reduction in a characteristic of the TFT. To solve such a problem, a technique disclosed in Japanese Patent Application Laid-open No. Hei 7-1883540 is one performed by adding a metal element having catalysis, such as nickel, in crystallization of an amorphous semiconductor film, and orientation property of the crystal orientation can be improved to be a single direction, in addition to an effect of decreasing a heating temperature required for the crystallization. When a TFT is made from a crystalline semiconductor film produced by this method, a reduction in a subthreshold coefficient (S value) and improvements of a static characteristic and a dynamic characteristics become possible in addition to an improvement of electric field effect mobility.
However, since a metal element having catalysis is added, there is such a problem that the metal element is left in the inner portion or the surface of the crystalline semiconductor film, and thus a characteristic of an obtained TFT is varied. One example is increase of an off current and there is such a problem that a variation between the individual TFTs is caused. That is, the metal element having catalysis to crystallization conversely becomes unnecessary once the crystalline semiconductor film has been formed.
Gettering using phosphorus is effectively used as a method of removing such a metal element from a specific region of the crystalline semiconductor film. For example, phosphorus is added to a source and a drain regions of a TFT and then heat treatment is performed at 450 to 700° C., whereby the metal element can be easily removed from the channel forming region.
Phosphorus is implanted to the crystalline semiconductor film by an ion dope method (which is a method of dissociating PH3 or the like with plasma and accelerating ions of PH3 by an electric field to implant it into a semiconductor, and a method in which ion mass separation is not basically performed). A concentration of phosphorus required for gettering is 1×1020/cm3 or higher. Addition of phosphorus by the ion dope method causes the crystalline semiconductor film to be amorphous. However, when the concentration of phosphorus is increased, a problem in which recrystallization by later anneal is hindered is caused. Also, since the addition of high concentration phosphorus causes an increase in a processing time required for doping, a problem in which throughput in a doping process is decreased is caused.
Further, a concentration of boron required for inverting a conductivity type is 1.5 to 3 times higher than that of phosphorus added to a source region and a drain region of a p-channel TFT. Thus, a problem in which resistances of the source region and the drain region are increased according to difficulty of recrystallization is caused.